Three dimensional positioning systems are commonly used when testing an aircraft to meet certain flight requirements as well as for certain flight and noise certifications. These positioning systems provide information and data regarding the aircraft position relative to other locations and factors during the test. Some of these other factors may include aircraft velocity, acceleration and orientation.
Flight tests and certification requirements may be made internally by the developer or purchaser of the aircraft, or required by governmental agencies such as those of the Federal Aviation Administration or the military. These tests and certification requirements are often custom flight tests, being specifically configured for the aircraft being evaluated, the test being performed, and the location or range of the test, among other factors.
The flight test applicant is typically required to show certain flight profile data in three-dimensional space. This data may include height above ground and distance from take-off point and/or the point in space at which a simulated engine failure occurs. In addition to the flight profile height and distance data, airspeed (velocity), rate of climb, engine power and take-off weight, among others must also be documented. Other test requirements may also be necessary. For example, the Federal Aviation Administration places strict wind limitations on flight tests in addition to requiring the flight profile data to be demonstrated over a range of density altitudes.
In general, flight tests and certifications and particularly, certain Federal Aviation Administration certification flight tests, require or are more easily accomplished, with the availability of highly accurate three-dimensional aircraft positioning and tracking data. In addition, these tests and certifications could greatly benefit from position and velocity feedback to the flight crew. This feedback, in the form of guidance cues could, for example, be used to correct any deviations from the required test flight plan.
Aircraft test programs, such as Federal Aviation Regulation Part 36, Appendix H, "noise certification" could also be greatly enhanced through the use of a highly accurate three-dimensional flight data and guidance system. In this particular certification test program, three precision flight profiles are required: level fly over; approach to landing; and the six degree approach to landing. Historically, the six degree approach to landing has been the most difficult to perform within the regulatory specifications. However, each of these tests requires the flight crew to accomplish difficult flight maneuvers, often with little ground or instrument positioning support.
Another area of flight testing that requires high accuracy three-dimensional aircraft position and velocity data is with instrument flight rules systems certification testing. Federal regulations are currently changing to allow use of global positioning systems navigation equipment to aid in instrument flights. Certification of the equipment that a pilot uses to stay on his assigned precision approach or departure flight path will require a highly accurate and efficient testing system to prove the applicant's product is capable of meeting the Federal Aviation Administration guidelines.
Some of these precision approach test paths are being designed which resemble long funnels having several turns, constantly decreasing in cross sectional area as they near the runway threshold. New regulations have been proposed and are under review to further tighten the existing `funnels` to accommodate increasing air traffic. As more accurate navigation systems are currently being developed, portable and cost effective flight checking and testing systems of greater accuracy are needed for certification of these navigational aids.
Existing three dimensional positioning and data recording systems capable of custom flight test plans and requirements include laser and encoding optical theodolite systems, grid cameras, as well as microwave trisponder systems integrated with a radar altimeter. These systems are bulky, expensive to operate and are relatively inaccurate.
Another disadvantage of these positioning and data recording systems is that they provide little or no, real-time, three dimensional position feedback or guidance to the flight crew. Any guidance cues available to the flight crew from these systems are relatively inaccurate and tend to induce pilot oscillation due to the low data update rates and high latency times. Thus, current cuing information is typically not useful for positioning and guidance of the aircraft by the flight crew or others on the ground.
Yet another disadvantage of these positioning and data recording systems is their need for a large flight test range area. Additionally, flight test range location choices were limited by system component geometry and line-of-site requirements. Temperamental performance of the equipment due to the ambient environment, including changes in ambient temperature and multipath effects also contributed to rejected data runs. All of these factors combined to create extremely inefficient flight testing activity using these positioning and data recording systems.
Alternative flight evaluation methods using experienced engineering test pilots and aircraft spotters to assist the pilots and confirm whether standards were met have also been employed. These methods are very inefficient, lack accuracy, and raise safety concerns. Additionally, these methods are limited to relatively clear skies and less dynamic maneuvers where the aircraft's flight instruments remain reliable.
More recently, differential global positioning systems have become available for accurately determining object position in three dimensions. These system are based on global positioning systems. Global positioning systems, or GPS, utilize position information signals that are broadcast from a constellation of satellites maintained, at least in part, by the United States government.
The GPS satellites currently broadcast position information signals on two frequencies; L1 (1575.4 MHz) and L2 (1227.6 MHZ). The L1 frequency carrier is currently modulated by two codes, the course acquisition code and the precision code. The L2 frequency carrier is currently modulated with only the precision code. The precision code is encrypted and only available to the military and authorized users. The course acquisition code is available to civilian users of GPS equipment. The accuracy of a course acquisition code GPS receivers is typically lower than that of precision code GPS receivers. Used alone, GPS is of little use in precision flight test applications.
Differential global positioning systems, or DGPS, work by installing one GPS receiver on the position to be analyzed and a second GPS receiver at a control point or reference location. The data from both the desired position and control location receivers are then merged. The resulting DGPS information yields very high position data accuracy in three dimensions. The data merging process can occur real time or in a post processing fashion.
To achieve real time differential global positioning, a first GPS receiver, denoted a reference station is located at the reference location. This reference GPS receiver compares its known location to the currently determined location generated from the latest GPS satellite information broadcast. The reference station develops correction factors that can then be broadcast or otherwise sent to other nearby GPS receivers known as rover(s) that are not at fixed control points. When these correction factors are applied by the rover receivers in a timely fashion, the positional accuracy data in all three dimensions is greatly improved.
Sending or otherwise communication the differential correction from the reference receiver to the rover receivers(s) requires some form of data communication. Since a rover may be moving, radio transmissions may be used. Radio modems that can reliably transmit this type of data should be equipped with forward error correction, an error checking technique. Currently, DGPS receivers capable of accuracy sufficient for flight testing are not available with radio data link modems. The use of radio transmissions also presents a reliability challenge to a system utilizing DGPS reliability.
Although DPGS provides very high positional accuracy, DGPS is currently not yet available for flight requirement and certification testing. Additionally, these systems are not configured to provide the flight or ground crew with needed guidance cues and tracking information. Needed information may include: aircraft position, direction, velocity and acceleration referenced to a selected coordinate system which can be used to correct a flight onto a desired flight path or to confirm the testing. Further, any flight guidance cues must have sufficiently high accuracy, sufficiently low latency and sufficiently high update rates such as to prevent pilot induced oscillation.
For the foregoing reasons, there is a need for an apparatus and method that can provide flight and ground crews with highly accurate aircraft positioning and velocity data as well as providing guidance and tracking information. There is also a need that the apparatus and method be portable, enabling flight testing to take place at a test range that can be established quickly and over most terrains. There is also a need for such a system that is relatively simple to operate and is inexpensive.